I have a 1994 4.0 4X4. 291,000 miles.


Currently the truck often spits and sputters a bit at idle, then more specifically under normal acceleration and going uphill causing the truck to shake and occasionally backfire a bit. I receive no Check Engine lights, and sometimes the truck will seemingly run fine for a time then go right back to spitting and sputtering. At steady speeds between 45-65 mph on level ground the truck often shows no symptoms.


The truck had all it's plugs and wires replaced 3 years ago. I changed the fuel filter on it a few days ago hoping that was my problem but that changed nothing.


Any suggestions or tips for trouble shooting? I apologize for another 1994 4.0 thread but after reading through several I didn't seem to find any with problems that reflected mine exactly.


Any help is greatly appreciated.

 

It sounds like you may have high resistance in your ignition system.

My standard diagnostic tip with something like your problem would be pull the spark plugs do a engine compression test on all cylinders to verify you have a good engine.

I say do a compression test because of the high mileage and it is easy to do so with the plugs removed.

inspect the spark plugs and measure the resistance of each one and check for signs of a high voltage leak to ground.

measure the resistance of each plug wire and the ignition coil.

If you do not find anything wrong with the compression, plugs, wires, or coil reinstall the parts making sure to use dielectric grease on the spark plug wires.

let us know once you get this far and still need help and we can go from there.

 

Thank you for the tips, it will be a few days before I get a chance to play around with anything due to life happenings.


I forgot to add in my original post I replaced the MAF sensor a year ago due to somewhat similar symptoms, which fixed things at the time. I have not cleaned or checked the MAF sensor in regards to this go around because my check engine light has yet to come on (it always did a year ago) and the fact that it was replaced not long ago.

 

UPDATE:


The plugs were finally pulled today. A compression test showed #1 Cylinder at 162 psi, #2 165 psi, #3 150 psi, #4 150 psi, #5 150 psi, #6 165 psi. Note: upon removal of the #5 plug it was noticed the plug was wet with gas with additional hard build up. This was the only plug in that condition, all other plugs had white build up and were dry. All plugs were brushed and cleaned. The #5 plug was also seen giving off a spark when turning the motor over.


After the plugs were cleaned all were put back in with the truck still running the same as described above. My gas mileage has also taken quite a hit by 50-60 miles less per gas fill up.

 

Did you measure the resistance of each spark plug and wire?

I wish you would have posted a picture of the spark plugs.

The whole point of removing the plugs was to measure them and measure the wires.

 

Replace spark plugs with Motorcraft or Autolite

Replace spark plug wires every 10 years or so

Compression is OK

There are two fuel lines in the engine bay
IN from fuel pump, goes to drivers side of fuel rail, between upper and lower intakes
OUT sends over pressure fuel back to the gas tank, this is on the FPR(fuel pressure regulator) located behind the alternator, passenger side of the engine
FPR will also have a Vacuum Hose attached
If FPR diaphragm leaks that vacuum line will suck fuel into the intake, sending MPG down and cause rough running like you describe
Remove this vacuum line and check for gasoline or gas smell.
You can remove hose and plug it, then start engine and see if gas drips out of the FPR's vacuum port.

Could also be a leaking injector, similar symptom, especially the wet spark plug
You can test for this by using the "Clear Flooded Engine" routine found in ALL fuel injection computers.
Turn key ON
Press gas pedal to the floor and hold it down ALL THE WAY
You are now in "Clear Flooded Engine", computer will NOT open injectors until you release the gas pedal.

So if no gas is going into the intake(injectors are OFF) then engine should NOT start when you crank it

Crank the engine
If it fires then gas is leaking in
If it just cranks then no gasoline is leaking in

Gas could be from FPR or Injector, so check FPR first.

 

Quick update: Life has been extra busy so I have not had a chance to trouble shoot some more based on the above suggestions. Thank you very much for the help!


I will update here soon after I look into what was listed above, hopefully by this weekend, and let you all know what I find.

 

Okay another quick update. I just did the check flooded engine routine. I turned my key on, pushed my gas pedal to the floor while holding the clutch in as well and turned the key. The truck turned over but did not crank. I did this several times to verify.

I plan to look at my FPR within the next few days hopefully.

 

"Clear flooded engine" is good to use on older engines when cold starting, pumps up the oil before releasing gas pedal and having engine start


Change the spark plugs, 4.0l eats them
Use regular copper, no platinum unless it is DOUBLE platinum, you have a Waste Spark system so single platinum is a waste of money

 

I had a few moments to check my FPR today. I pulled the slim black line off the bottom side of the FPR and checked for any signs of fuel and found none. I also ran the truck with it off and could not produce or see any signs of fuel.

 

At this point I plan to try new plugs. Any thoughts as to if they could be the entire culprit? What would cause the one wet plug?

 

A fuel fouled spark plug could be just that the plug is bad. it also could be a fuel injector dribbling fuel rather than spraying fuel.

Spark plugs are easy to damage if not handled and installed carefully.

Even if you handle your new box of plugs properly who knows what happened to them from the manufacture to your hands. for all we know the guys at the parts store could have been playing catch with them LOL

If you have read any of my post where I try and help people you will see over and over where I recommend basic diagnostics this is the same for your truck.

With almost 300000 miles on your truck I certainly would recommend a compression test on your truck. Low compression from a burnt valve may fire a air fuel mixture yet it may not do it completely allowing the spark plug to fuel foul.

Hopefully this is not the case with your engine but since your going to replace the spark plugs anyways why not run the test and know for sure?

If the compression is good and you do not see any physical damage to the plug and you measured the resistance of each plug and they are all close to the same, I would get a stethoscope and listen to each injector to make sure you hear a nice clean crisp click.

With the mileage you have on your engine It would not be surprising if your injectors need to be replaced. ( however if you get to the point where your considering that job let us know and there are a few test to confirm obviously bad or leaking injectors.

 

Way overdue post here. I changed the plugs and the truck now runs great. These plugs were only 3 years old but I suppose RonD was on the money when he said the 4.0 eats them. With the mileage my truck has it could "go" at any moment, but it doesn't smoke and just burns a little oil so I'm hopeful it might "eat" another pair of plugs before I have to do an overhaul. Next time I'll know to check the plugs earlier on in the process.


Thank you to EaOutlaw and RonD for your input and suggestions. I've been able to learn a lot about my motor by coming here. There's always something new to learn but I feel like I've got a good start to troubleshoot different things the next time my motor has any issues based on suggestions here at the forum. Thanks again!

 

I am glad you found the problem, I will say my very first words were I thought you had high resistance in the ignition system.

I bet if you had measured the plugs and wires like I asked you would have found and fixed the problem the very same day you asked for help.

It is not my intention to pick on you, but it is my intention to get you to gear up with some basic tools like a DVOM and practice with it before you may need it next time.

I would also get a Good hand held scanner, and a factory service manual and a computer safe 12 volt test light,if you do not already own them.

Good luck with your truck.

 

Thread bump here. My truck is up to 297,000 miles now and about about 2 weeks ago it started having the same symptoms as I described above. Things like sputtering and just rough running in general, especially when under load. I changed my plugs about 6,000 miles ago and that fixed my problems instantly.

Since the onset of these same symptoms I decided this time to go ahead and change my plug wires since they do have just over 50,000 miles on them and I did not replace them when I did my plugs a few months ago. However on my test drive with my new plug wires the same symptoms are still present.

Could my plugs have already gone bad again? They are basic copper Autolites. What else might be happening to cause these symptoms? Any thoughts or recommendations much appreciated.

 

I would pull out and clean the plugs, have a look at them in general

You will be burning some oil from valve guide seal leaks which can foul plugs faster.

Clean MAF sensor, once a year, major cause of bogging on 4.0l

Also add a can of Seafoam or similar injector cleaner to gas tank once a year, really smooths out my '94 4.0l, 300K +

 

I took my regular copper plugs out yesterday and replaced them with new Autolite Double Platinum plugs. The truck now runs strong and smooth. I’m hoping to get more than 7,000 miles out of these plugs. Below is a picture of my plugs after I pulled them with their cylinder number above them. As you can see both the 3 & 4 had oil on them at their midsections.